Referring to FIGS. 1A, 1B and 1C of FIG. 1, an optical excitation at 193 nm (6.5 eV) is near ‘resonantly’ tuned to optically excite Photo Active Groups (PAGs) contained in a layer of photoresist (resist). Secondary electrons do not play any significant role in the optical excitation of PAGs at 193 nm. Optical lithography control of a photoresist profile can be maintained by both minimizing back reflections (for example, standing waves) from an underlayer 3 (such as an optical planarizing layer (OPL)) into a resist layer 1 through n,k (real and imaginary parts of the reflection) matching of an anti-reflection (AR) coating interface layer 2. In conventional practice the AR coating can be used to maximize the optical contrast at the interface layer 2 and promote acid-base chemical interaction. In general, acid bases can typically be located at the interface and a controlled amount of reflection (back into the resist layer 1) can be used to clear out the bottom of the resist profile and thus reduce a required photolithographic dose. It can be noted, however, that with EUV excitation of the resist/interface/underlayer the light travels through the layers without experiencing any appreciable reflection at the top or bottom surfaces of the interface layer.
As can be seen in FIG. 1B, there are electron interactions after incident 92 eV photons (13.5 nm) generate photoelectrons during ionization events. Due to photoionization PAG excitation can occur by secondary electrons having energies of less than 10 eV. As is shown in FIG. 1C, the electron inelastic mean free path defines the travel extent of secondary electrons across the resist polymer. The secondary electrons can travel some tens of nanometers and can traverse from the resist layer 1 into the interface layer 2.
In U.S. Pat. No. 8,968,989 B2, “Assist layers for EUV lithography”, Ouattara et al. disclose spin-on assist layers for EUV which contain at least 0.01% of metal and up to 70% of metal by weight. The spun-on assist layer can include both silicon and metal oxides and nitrides. The assist layer is generally described as an underlayer that is spun-on below the photoresist layer to enhance patterning with EUV. As the photoresist layer is exposed to EUV radiation the assist layer absorbance is said to be modulated as a source of secondary electron generation.
In U.S. Pat. No. 8,257,910 B1, “Underlayers for EUV lithography”, Guerrero et al. disclose spin-on underlayer compositions for EUV formed of thermally cross-linkable materials. In an embodiment metal and/or silicon atoms can be provided in the composition. Preferred metals are said to include those selected from the group consisting of the early transition metals (Groups MB and IVB), with Ti, Zr, and Hf being particularly preferred. The total metal and silicon atoms present in the composition could be from about 4% to about 80% by weight metal. As the photoresist layer is exposed to EUV radiation it is said that the underlayer layer absorbance is modulated as a source of secondary electron generation.